1. Field of the Invention
The present invention relates generally to a control scheme. More particularly the present invention relates to a method and apparatus for more accurately and stably limiting critical variables associated with a process such as those including turbomachines such as a turbocompressor, steam turbine, gas turbine, or expander.
2. Background Art
The safe operating regime of a turbocompressor is constrained by the machinery and process limitations. A turbine-driven turbocompressor is generally bound by upper and lower limits of a turbine operating speed, a surge line, a choke limit, high discharge or low suction pressure bounds, and/or a power rating of the turbine. Limit control is used to keep the turbocompressor from entering an operating regime that is not considered safe, is unacceptable from a process standpoint, or undesirable for any reason. Limit control, also referred to as constraint control, is defined as a control strategy that will take action to avoid operating in these undesirable operating regimes, but only takes action when there is a tendency or danger of operating therein. Take, for example, a turbocompressor's discharge pressure that is to be constrained to remain at or below a set point, psp. When the turbocompressor's discharge pressure is below psp, no particular action is taken by the limit control system to adjust psp. Only when the turbocompressor's discharge pressure reaches or exceeds psp is control action taken. Limit control strategies differ from ordinary control strategies in that: ordinary control strategies take measures to keep the process variable at its set point at all times (generally speaking), keeping the process variable from dropping below its set point as well as keeping it from exceeding its set point; limit control strategies are brought to bear only when a limit variable crosses its set point. On one side of its set point, the limit control scheme is not in effect.
Often, a rigid limit set point exists where a safety system, associated with the machinery or process, causes the machinery to shut down, or a relief valve to open, etc. The process control system, on the other hand, makes use of soft set points. A soft set point is separated from its associated rigid set point by a safety margin. Minimization of the safety margins results in an expanded operating envelope.
Advanced antisurge control systems have been applied very successfully in many applications to prevent the turbocompressor from damages due to surge. In U.S. Pat. No. 4,949,276, a method of antisurge control is disclosed using a speed of approach to surge to increase the safety margin. Once the compressor's operating point has reached the controller's surge control line, closed loop control attempts to prohibit surge by opening an antisurge valve. Open loop control is disclosed in U.S. Pat. Nos. 4,142,838 and 4,486,142. Here, an open loop control line is located toward surge from the surge control line. If closed loop control is unable to keep the compressor's operating point from reaching this open loop control line, an open loop control action will cause the antisurge valve to open as quickly as possible a predetermined increment.
A scheme similar to that just described for antisurge control was patented in U.S. Pat. No. 5,609,465 for overspeed control in turbines. Here, a steam valve is closed a predetermined increment as quickly as possible by an open loop control action.
Such advanced control schemes have not been applied for other constraints imposed on turbomachinery. Surge and overspeed are known to cause process upsets, but are somewhat unique in their ability to cause damage and destruction to the turbomachinery and adjacent equipment, and even to be dangerous to personnel. In the past, there was no motivation to apply these advanced techniques, along with their complexity, to other constrain control problems. In fact, common understanding taught that an open loop action would cause process upsets, thereby teaching away from the use of these advanced control schemes that resulted in what were considered severe reactions to process events causing a control action. Recently however, competitive conditions and political-economic-environmental issues such as the restriction on carbon dioxide emissions have resulted in reconsidering control strategies to squeeze the last percentage of efficiency from processes, and expand the operating envelope of the process as much as possible.
For instance, because of a process upset or a change in operating conditions, a turbocompressor's suction pressure may drop below atmospheric pressure, a condition that can cause air to be entrained in a hydrocarbon being compressed. Or the turbocompressor's interstage pressure may exceed a maximum pressure rating for the machinery casing or process vessels. Present-day control systems typically utilize a secondary-variable closed-loop control scheme to constrain the turbomachine's operating point within predetermined bounds. When a limit-control variable reaches its set point, control is bumplessly transferred from primary variable control to secondary variable limit control and the manipulated variable of the turbomachine is adjusted to bring and/or keep the offending limit-control variable within acceptable limits. Due to excessive dead times or large time constants in the overall system, traditional PID based constraint control actions may sometimes be inadequate to prevent an excursion of a critical process variable into a restricted region caused by a process upset. Moreover the set points configured for limit control are fixed. Therefore, limit control is initiated only if a variable crosses its predetermined limit, that is, a measurable error is incurred. Increasing the gains of the controller may not mitigate the problem due to the overall system's sluggishness (long dead times or large time constants). The best solution to this situation is to configure the control system with conservative safety margins. This invariably contracts the available operating zone of the turbocompressor. The consequence of such a control approach is a decrease in the turbocompressor's throughput with its associated significant impact on plant production.
There is, therefore, a need for a limit-control strategy that effectively and stably results in the constraining of limited variables, while bumplessly transferring between primary variable control and constraint variable control.